Summary Cryptosporidium remains an important opportunistic pathogen in AIDS patients who do not have access to HAART treatment. This parasite infects the gastrointestinal epithelium in humans; infection in AIDS patients often leads to life-threatening illness. Cryptosporidium is also a common cause of diarrhea in young children in developing countries. Indeed, infection with Cryptosporidium shows significant association with mortality in young children and appears to predispose children to enteric infection with lasting deficits in age-appropriate body growth and cognitive development. The underlying molecular mechanisms are unclear. The primary infection site of Cryptosporidium in human is the small intestine, one of tissues in the body that regenerates the most quickly. In our preliminary study, we observed that Cryptosporidium infection is limited to the villi (mainly enterocytes) of intestinal epithelium. Nevertheless, infection causes a significant decrease in intestinal stem cell (ISC) numbers (as well as the expression levels of several ISC markers). We also identified a panel of genes whose expression levels are significantly altered in enterocytes following infection. Some of these genes code important components of the signaling pathways in the enterocyte-ISC network and thus, may be involved in Cryptosporidium-induced ISC dysfunction. Therefore, we hypothesize that Cryptosporidium infection in the villi of intestinal epithelium impedes ISC function in the crypts through changes of signaling pathways in the enterocyte-to-ISC network. We will use in vitro, ex vivo, and in vivo infection models and complementary biochemical, molecular, and morphologic approaches to characterize the dysfunction of ISC populations (ISC plasticity) during Cryptosporidium infection (Aim 1) and identify the signaling pathways pertinent to ISC dysfunction induced by infection (Aim 2). With the completion of these aims, we will have determined the ISC plasticity during Cryprosporidium infection and identified which signaling pathways are specifically affected by infection leading to ISC dysfunction. These outcomes will establish a foundation on which to build long-term, mechnistic studies to define the pathogenesis (particularly the long-lasting effects) of Cryptosporidium infection in the intestine.